Remove zest from Lemons and save juice for use later.
Place zest in food processor and add Sugar. Run until sugar and zest are well combined and fine.
In mixer cream Butter and then add the lemon/sugar and Flour. Add Egg Yolks one at a time. Add Lemon Juice and Kosher Salt. Mix well.
Pour Mixture into a saucepan and cook over low heat for about 10 minutes stirring constantly until thickened. Remove from heat and let cool. Keep refrigerated until ready to use.